It is known that blast furnaces are charged via a lock which is alternately put in communication with the atmosphere during its filling and with the interior of the furnace during the flow-off of the charging material. For this purpose, the flow-off orifice and the charging orifice of the lock are equipped with sealing shutters. The shutters typically include a soft peripheral gasket and are movable between a first position, wherein the gasket is laid against an annular seat surrounding the orifice, and a second position, wherein the gasket is set apart from the annular seat.
Each of these sealing shutters is actuated by a hydraulic jack or the like, the thrust of which is of the order of a few tons, depending on the size of the shutter. In addition to this hydraulic force, in the closed position, the shutter is subjected to the force resulting from the pressure differences prevailing on either side of the shutter. In fact, an atmospheric pressure prevails on the same side as the seat of each shutter, while on the opposite side the shutters undergo, in their closing direction, the pressure which prevails inside the furnace. Depending on the surface of the shutter, this pressure difference may result in a force of the order of 30 tons. Because the gaskets of the shutters are made of soft material, the seat risks biting into the gasket under the influence of this high force, to form there an annular depression conductive to deposits of dust and dirt and accelerating the wear on the gasket. Although it would be possible to cancel the hydraulic thrust on the shutters when they are in their closed position, nevertheless this measure would scarcely solve the problem because the hydraulic force is negligible as compared with the thrust arising as a result of the pressure difference.